Prevent dangling pointer

Regression introduced with 8abfa8082997bf59cd20aba2bd86de0eb3f88279

 - _dispose might be called multiple times, so g_list_free_full might have
   been called on an already freed list
 - emit_notifications left a dangling pointer which caused _dispose to crash

Signed-off-by: Jens Georg <mail@jensge.org>
Unref root device in ACL server handler

We have an indirect reference through handler->service, so no need to
keep around the one added through g_object_get.

This fixes the case where test-bugs randomly ran into timeouts because it
subscribed to the wrong service on the wrong (dangling) device.

Signed-off-by: Jens Georg <mail@jensge.org>